Gires-Tournois interferometer with faraday rotators for optical signal interleaver

ABSTRACT

A GTIFR interferometer, for use in an interleaver or in a deinterleaver, wherein the GTIFR interferometer includes a Gires-Toutnois interferometer with a 45 degree Faraday rotator between the mirrors of the Gires-Tournois interferometer and further includes a 22.5 degree Faraday rotator in the light path to the Gires-Tournois interferometer and an interleaver or deinterleaver that contains one GTIFR. A dispersion compensated GTIFR interleaver includes a second Gires-Tournois interferometer for providing chromatic dispersion compensation.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional patent application of a U.S. patentapplication Ser. No. 09/929,875, filed Aug. 13, 2001, now allowed, thecontent of which is hereby incorporated herein by reference. Thisapplication further relates to a co-pending U.S. patent application Ser.No. 09/874,925, filed Jun. 04, 2001, the content of which is also herebyincorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to the field opticalcommunications systems. More particularly, the invention relates tointerface devices such as interleavers and de-interleavers that are usedfor interfacing between portions of Dense Wave Division Multiplexed(DWDM) systems that operate at channel spacings differing by a factor oftwo, between, for example, portions operating at 50 GHz per channel andportions operating at 100 GHz per channel.

BACKGROUND OF THE INVENTION

[0003] As DWDM optical communications technology has progressed, thechannel spacing has changed over a number of years from 200 GHz to 100GHz to 50 GHz per channel. When a communications system is in theprocess of being upgraded from say 100 GHz per channel to 50 GHz perchannel, it may be expedient to retain some older equipment in thesystem, for example, older equipment that was designed for use at 100GHz per channel. The older equipment can be retained in the upgradedsystem by using interleavers and deinterleavers to interface between theolder equipment and the newer equipment.

[0004] An interleaver combines an optical signal containing evenchannels with an optical signal containing odd channels. A 50 GHzinterleaver, for example, combines an optical signal containing a set ofeven channels having 100 GHz spacing with an optical signal containing aset of odd channels having 100 GHz spacing and produces an outputoptical signal containing the set of even channels and the set of oddchannels with 50 GHz spacing per channel.

[0005] A deinterleaver reverses the process of the interleaver. A 50 GHzdeinterleaver, for example, separates even channels from odd channels toproduce two output signals, an output signal containing the set of evenchannels having 100 GHz channel spacing and an output signal containingodd channels and having 100 GHz channel spacing.

[0006] The general principle of the interleaver is based on theinterference of two light beams. The interference creates a periodicrepeating output as different integral multiples of wavelengths passthrough the device and the desired channel spacing of the interleaver isset by controlling the fringe pattern. Manufacturers today usefused-fiber Mach-Zehnder interferometers, liquid crystals, birefringentcrystals, Gires-Tournois interferometers (GTI) and other devices tobuild interleavers and deinterleavers.

[0007] Of these, the GTI based interleaver and deinterleaver have manyadvantages over the rest. For example, a GTI based interleaver has verylow insertion loss, has uniform response over a wide range ofwavelengths (flat-top spectrum), and has minimal polarization dependenceeffect.

[0008] Chromatic dispersion must be considered for 10 Gbit/s and nextgeneration 40 Gbit/s systems. Chromatic dispersion requirements for thehigher bit rate systems are extremely tight. While there are currentlymany technologies being pursued for use in interleaver products, thedispersion performance will probably be a critical factor in determiningwhich technology will be successful. To be successful, the interleavermust not only have a low dispersion value at the center ITU wavelength,but over the full useful passband of the device (i.e. the dispersionshould not reduce the usable passband). Unfortunately, the GTI basedinterleaver has a very large dispersion of up to 70-200 ps/nm for a 50GHz interleaver and up to 250-800 ps/nm for a 25 GHz interleaver.

[0009] U.S. Pat. No. 6,169,604, entitled “NONLINEAR INTERFEROMETER FORFIBER OPTIC DENSE WAVELENGTH DIVISION MULTIPLEXER UTILIZING A PHASE BIASELEMENT TO SEPARATE WAVELENGTHS IN AN OPTICAL SIGNAL”, issued to Cao,discloses a multiplexer that includes two non-linear interferometers,wherein each nonlinear interferometer (NLI) is a GTI with an internalλ/4 wave-plate and an external λ/8 wave-plate (FIGS. 8 and 9 in U.S.Pat. No. 6,169,604).

[0010] In co-pending Patent Application No. 09/874,925, entitled“OPTICAL SIGNAL INTERLEAVER AND DEINTERLEAVER DEVICES WITH CHROMATICDISPERSION COMPENSATION”, incorporated herein by reference, the presentinventor discloses a dispersion compensated interleaver and a dispersioncompensated deinterleaver, each of which includes an NLI and a GTIdispersion compensator.

[0011] In an NLI, the angular alignment of the c-axis of the wave-platesrelative to the direction of polarization is both critical anddifficult. The correct angular alignment of the direction of the c-axisrelative to the direction of polarization of the light beam entering thewave-plate is 45° for both the λ/4 wave-plate and the λ/8 wave-plate.Misalignment of as little as 1°, that is to say rotation of a wave-plateby as little as 1° around the direction of the beam from 45° to 46° orto 44°, causes serious distortion of the spectrum shape and group delayof the interleaver (or deinterleaver). Some examples of the distortionsarising from misalignment of the λ/8 plate will now be presented.

[0012] Misalignment degrades the isolation between channels. Forexample, a misalignment of the λ/8 wave-plate by 1° reduces theisolation between even channels and adjacent odd channels by 10%.

[0013] Misalignment distorts the group delay. FIG. 1 shows the groupdelay of a perfectly aligned 50/100 GHz NLI based interleaver (ordeinterleaver). FIG. 1 applies to both odd and even channels. FIGS. 2and 3 show group delay for odd and even channels, respectively, for thesame NLI based device as in FIG. 1, but with misalignment of the c-axisof the λ/8 wave-plate of 1°, from 45° to 46°. The distortion of thegroup delay in FIGS. 2 and 3 as compared to FIG. 1 is apparent. Notethat the distortion of the group delay is different for odd channelsthan for even channels. Such serious asymmetric distortions of the groupdelay make it very difficult to compensate for chromatic dispersion.

OBJECTS AND ADVANTAGES

[0014] It is an object of the present invention to provide aninterferometer for use in an interleaver and in a deinterleaver, aninterferometer which separates or combines signals for odd and evenchannels and which has a group delay characteristic that is free ofdistortion arising from misalignment of phase shifting componentsrelative to the direction of polarization of light entering theinterferometer.

[0015] It is an object of the present invention to provide an opticalinterleaver and deinterleaver in which the group delay is not subject todistortion arising from angular misalignment of optical phase shiftercomponents around an axis defined by the direction of the optical beam.

[0016] It is a further object of the present invention to provide aninterleaver and deinterleaver in which the isolation between channels isnot subject to degradation arising from angular misalignment of opticalphase shifter components around an axis defined by the direction of theoptical beam.

[0017] It is a further object of the present invention to provide aninterleaver and deinterleaver in which a GTI is used as a dispersioncompensator, and in which the absence of angular misalignment allowsmore effective chromatic dispersion compensation.

SUMMARY

[0018] The objects and advantages of the present invention are providedby an interferometer that is a combination of a Gires-Tournoisinterferometer with Faraday rotators (hereafter GTIFR). The GTIFR isintended for use in a deinterleaver or deinterleaver.

[0019] The GTIFR in accordance with the present invention is aGires-Tournois interferometer with a 45-degree Faraday rotator betweenthe mirrors of the Gires-Tournois interferometer and a 22.5-degreeFaraday rotator in the light path of light entering and leaving theGires-Tournois interferometer. The Faraday rotators are preferablygarnets, though any other type of Faraday rotator may be used.

[0020] The structure of the GTIFR includes a Gires-Tournoisinterferometer that has a partially reflective mirror optically coupledto a highly reflective mirror. The mirrors are parallel and separated bya fixed distance d. In the cavity between the mirrors there is a45-degree Faraday rotator in the light path. The partially reflectivemirror provides a port for light to enter and leave the Gires-Tournoisinterferometer. Outside the Gires-Tournois interferometer, in the pathlight entering and leaving the Gires-Tournois interferometer, there is a22.5-degree Faraday rotator.

[0021] In the operation of the GTIFR, as used in the single GTIFPinterleaver or deinterleaver of the present invention, plane polarizedlight containing signals for odd and even channels passes through the22.5 degree Faraday rotator and becomes circularly polarized with 22.5degree phase difference. The light then is then reflected from theGires-Tournois interferometer with phase change that is a function ofthe GTI and of the 45 degree Faraday rotator. The light then passesthrough the 22.5 degree Faraday rotator for a second time, where thephase characteristic is again changed.

[0022] An interleaver in accordance with the present invention requiresonly one GTIFR. Likewise a deinterleaver in accordance with the presentinvention requires only one GTIFR.

[0023] A GTIFR in accordance with the present invention, when used in adeinterleaver in accordance with the present invention, receives opticalsignals that are plane polarized in one direction and that containsignals for a set even channels and for a set of odd channels. Thesignals are reflected by the GTIFR and then enter a polarization beamsplitter. A signal containing one set of channels is reflected from thepolarization beam splitter and a signal containing the other set ofchannels is transmitted by the polarization beam splitter.

[0024] A GTIFR in accordance with the present invention, when used in aninterleaver in accordance with the present invention, receives planepolarized optical signals for a set of even channels and for a set ofodd channels, the direction of polarization for the even channels beingperpendicular to the direction of polarization of the odd channels. Thesignals are reflected from the GTIFR and enter a polarization beamsplitter where both sets of channels are transmitted by the beamsplitter.

[0025] As disclosed in co-pending application Ser. No. 09/874,925, aGires-Tournois interferometer can be used to compensate for chromaticdispersion in a deinterleaver or interleaver.

[0026] In a dispersion compensated deinterleaver in accordance with thepresent invention, the even channel and odd channel signals pass througha Gires-Tournois interferometer dispersion compensator before beingreflected by the GTIFR.

[0027] In a dispersion compensated interleaver in accordance with thepresent invention, the even channel and odd channel signals pass througha Gires-Tournois interferometer dispersion compensator after beingreflected by the GTIFR.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a graph of the group delay for a NLI in which the λ/4wave-plate and the λ/8 wave-plate are perfectly aligned.

[0029]FIG. 2 is a graph showing the group delay for odd channels for aNLI in which the λ/8 wave-plate is misaligned by 1°.

[0030]FIG. 3 is a graph showing the group delay for even channels for aNLI in which the λ/8 wave-plate is misaligned by 1°.

[0031]FIG. 4 is a block diagram illustrating the function of adeinterleaver.

[0032]FIG. 5 shows the optical path of a Mach-Zehnder interferometerbased deinterleaver.

[0033]FIG. 6 shows optical path of a polarization-split interferometerbased deinterleaver.

[0034]FIG. 7a is a graph showing phase difference Δφ(λ) versus λ for anideal deinterleaver.

[0035]FIG. 7b is a graph showing the output of odd channels from anideal deinterleaver.

[0036]FIG. 7c is a graph showing the output of even channels from anideal deinterleaver.

[0037]FIG. 8 is a schematic representation of the function of θ-degreeFaraday rotator.

[0038]FIG. 9 shows a cross section of a Gires-Tournois interferometer(GTI).

[0039]FIG. 10 is a graph of the phase response of a GTI.

[0040]FIG. 11 is a cross section of a GTIFR in accordance with thepresent invention.

[0041]FIG. 12 is graph of the right-circularly polarized wave phasedelay ψ_(R)(λ) and left-circularly polarized wave phase-delay ψ_(L)(λ)for a GTIFR.

[0042]FIG. 13 is a graph of phase difference Δφ(λ) between theright-circularly and left-circularly polarized waves for a GTIFR.

[0043]FIG. 14 is a graph of the signal output of odd and even channelsof a GTIFR based deinterleaver.

[0044]FIG. 15 is a schematic diagram of a GTIFR based deinterleaver inaccordance with the present invention.

[0045]FIG. 15A is a schematic diagram of a GTIR based interleaver inaccordance with the present invention.

[0046]FIG. 16 is a schematic diagram of a GTIFR based deinterleaver withdispersion compensator in accordance with the present invention.

[0047]FIG. 17 is a schematic diagram of a GTIFR based interleaver withdispersion compensator in accordance with the present invention.

[0048]FIG. 18 is a graph of the group delay τ(λ) of a GTI dispersioncompensator.

[0049]FIG. 19 is a graph of the group delay of the GTIFR based 50 GHzinterleaver in accordance with the present invention.

[0050]FIG. 20 is a graph of the dispersion of the GTIFR based 50 GHzinterleaver in accordance with the present invention.

[0051]FIG. 21 is a graph of the group delay of the GTIFR based 50 GHzinterleaver with a GTI dispersion compensator in accordance with thepresent invention.

[0052]FIG. 22 is a graph showing the dispersion of the GTIFR based 50GHz interleaver with a GTI dispersion compensator.

DETAILED DESCRIPTION

[0053]FIG. 4 shows an optical deinterleaver and associated input andoutput signals. In FIG. 4, a deinterleaver 10 receives an input opticalsignal 12 containing a set of even channels and a set of odd channels,λ₁, λ₂, λ₃ and so on, and provides an output optical signal 14containing the set of odd channels λ₁, λ₃ and so on, and provides aseparate output optical signal 16 containing even channels λ₂, λ₄ and soon.

[0054]FIG. 5 shows a schematic block diagram of a Mach-Zehnder basedde-interleaver 20. An input light beam 22 enters beam splitter 24. Thebeam splitter 24 outputs two light beams 26 and 28. The power of thelight beam 26 is approximately equal to the power of the light beam 28.The light beams 26 and 28 enter the phase shifter 30 and emerge as lightbeams 32 and 34 between which there is a phase difference. The two lightbeams 32 and 34 combine and interfere in the interferometer 36. From theinterferometer 36, two output signals are obtained, output signal 38containing odd channels, and output signal 40 containing even channels.

[0055] In the present invention, instead of using a beam splitter, aplane-polarized wave is decomposed into two orthonormally polarizedwaves between which there is a phase difference that is introduced by aninterferometer. The phase difference is dependent on the wavelength ofthe optical signal and is a periodic function of wavelength, having aperiod that is equal to one or two channel spacings. The components ofthe orthonormally polarized waves which pass through a polarizationbeam-splitter (PBS) interfere and become output signals for oddchannels. The components, which are reflected from the PBS, interfereand become output signals for even channels.

[0056] In the following derivation, there will be derived values of theparameters needed for making a GTIFR based deinterleaver or interleaverin accordance with the present invention, including reflectivity of theGTI partially reflective mirror and rotating angle values for theFaraday rotators.

[0057]FIG. 6 shows a diagram of a deinterleaver 42. {right arrow over(E)} is the polarization of a wave that has unit power and propagates inthe positive z direction. {right arrow over (E)} is parallel to thepositive y-direction. The polarization {right arrow over (E)} can beexpressed as $\begin{matrix}{\overset{\rightarrow}{E} = {\sum\limits_{i = 1}^{2}{a_{i}{{\overset{\rightarrow}{e}}_{i}.}}}} & (1)\end{matrix}$

[0058] The set of basis vectors {right arrow over (e)}_(i) isorthonormal, i.e., $\begin{matrix}{{e_{i}e_{j}^{*}} = {\delta_{ij} = {\langle\left. \begin{matrix}{1,{i = j}} \\{0,{i \neq j}}\end{matrix} \middle| . \right.}}} & (2)\end{matrix}$

[0059] Using Jones vectors, {right arrow over (E)} can be expressed as$\begin{matrix}{\overset{\rightarrow}{E} = {\begin{pmatrix}0 \\1\end{pmatrix} = {{a_{1}\begin{pmatrix}e_{11} \\e_{12}\end{pmatrix}} + {{a_{2}\begin{pmatrix}e_{21} \\e_{22}\end{pmatrix}}.}}}} & (3)\end{matrix}$

[0060] After the beams have passed through (or been reflected from) aninterferometer 44, the beams e₁ and e₂ have phase shifts φ₁ and φ₂,respectively, and equation (3) becomes $\begin{matrix}{{{a_{1}\begin{pmatrix}e_{11} \\e_{12}\end{pmatrix}}^{{\varphi}_{1}}} + {{a_{2}\begin{pmatrix}e_{21} \\e_{22}\end{pmatrix}}{^{{\varphi}_{2}}.}}} & (4)\end{matrix}$

[0061] In the following derivation, the discussion is limited to thesymmetric case where α₁=α₂=α. When the beams travel to the polarizationbeam splitter (PBS) 46, the p-wave (polarization parallel to they-direction as in the input light beam) passes through the PBS 46 andthe s-wave (with polarization parallel the x-direction, i.e.,perpendicular to the polarization of the input light beam) is reflectedby the diagonal plane of the PBS 46. The p-wave 48, carrying the oddchannels, is output at one output port of the deinterleaver 42, and thes-wave 50, carrying the even channels, is output at another output portof the deinterleaver 42. The p-wave can be represented by$\begin{matrix}{a\quad {^{{- }\frac{\varphi_{1} + \varphi_{2}}{2}}\left( {e_{12}^{\frac{\Delta\varphi}{2}}e_{22}^{{- }\frac{\Delta\varphi}{2}}} \right)}\begin{pmatrix}0 \\1\end{pmatrix}} & (5)\end{matrix}$

[0062] and the s-wave can be represented by $\begin{matrix}{a\quad {^{{- }\frac{\varphi_{1} + \varphi_{2}}{2}}\left( {e_{11}^{\frac{\Delta\varphi}{2}}e_{21}^{{- }\frac{\Delta\varphi}{2}}} \right)}\begin{pmatrix}1 \\0\end{pmatrix}} & (6)\end{matrix}$

[0063] where Δφ=φ₁−φ₂.

[0064] For the circularly polarized basis, where $\begin{matrix}{{a = \frac{1}{\sqrt{2}}},} \\{{e_{11} = {{- e_{21}} = {- \frac{i}{\sqrt{2}}}}},{and}} \\{{e_{12} = {e_{22} = \frac{1}{\sqrt{2}}}},}\end{matrix}$

[0065] the p-wave is represented by $\begin{matrix}{^{{- }\frac{\varphi_{1} + \varphi_{2}}{2}}\cos \frac{\Delta\varphi}{2}} & (7)\end{matrix}$

[0066] and the s-wave is represented by $\begin{matrix}{^{{- }\frac{\varphi_{1} + \varphi_{2}}{2}}\sin \frac{\Delta\varphi}{2}} & (8)\end{matrix}$

[0067] For the plane-polarized basis, where, $\begin{matrix}{{a = \frac{1}{\sqrt{2}}},} \\{{e_{11} = {e_{12} = {e_{22} = \frac{1}{\sqrt{2}}}}},{and}} \\{{e_{21} = \frac{1}{\sqrt{2}}},}\end{matrix}$

[0068] the p-wave is represented by $\begin{matrix}{^{{- }\frac{\varphi_{1} + \varphi_{2}}{2}}\cos \frac{\Delta\varphi}{2}} & (9)\end{matrix}$

[0069] and the s-wave is represented by $\begin{matrix}{i\quad ^{{- }\frac{\varphi_{1} + \varphi_{2}}{2}}\sin {\frac{\Delta\varphi}{2}.}} & (10)\end{matrix}$

[0070] In order to make a deinterleaver (or interleaver) using theconfiguration as shown in FIG. 6, the phase difference Δφ should meetthe following requirements: the change of Δφ with wavelength must beperiodic, with the period being equal to one or two channel spacings,and the value of Δφ should be close to zero (or ±2 mπ, where m is aninteger) in one half of each period and close to π (or ±(2 m+1)π) in theother half of each period.

[0071]FIG. 7a is a graph showing the phase difference Δφ(λ) versus λ foran ideal case. Here Δφ(λ) is periodic with a period equal to two channelspacings.

[0072]FIG. 7b is a graph showing odd channel output versus wavelengthfor the ideal case.

[0073]FIG. 7c is a graph showing even channel output versus wavelengthfor the ideal case.

[0074] The interferometer and phase shifter are key optical componentsthat create phase difference Δφ. For a circularly polarized wave thephase shifter is a Faraday rotator (FR). A θ-degree FR rotatespolarization of a plane-polarized wave, which passes through the FR by θdegrees, as illustrated in FIG. 8. The input wave 52 is plane polarizedin a direction parallel to the y-axis. After passing through theθ-degree Faraday rotator or garnet 54, the output wave 56 is polarizedin a direction that is rotated θ degrees from the y-direction. Accordingto equation (3), the plane-polarized wave in the FR can be decomposedinto a right-circularly polarized component and a left-circularlypolarized component. The wave traveling in the positive z-direction isgiven by $\begin{matrix}{{\frac{1}{2}\begin{pmatrix}{- i} \\1\end{pmatrix}{\exp \left\lbrack {\left( {{\omega \quad t} - {\frac{2\pi}{\lambda}{nz}}} \right)} \right\rbrack}} + {\frac{1}{2}\begin{pmatrix}i \\1\end{pmatrix}{{\exp \left\lbrack {\left( {{\omega \quad t} - {\frac{2\pi}{\lambda}{nz}}} \right)} \right\rbrack}.}}} & (11)\end{matrix}$

[0075] Assuming that the right-circularly polarized wave sees an indexof refraction n₊ and the left-circularly polarized wave sees an index ofrefraction n, then, upon leaving the Faraday rotator, the wave isdescribed by $\begin{matrix}{{\frac{1}{2}\begin{pmatrix}{- i} \\1\end{pmatrix}{\exp \left\lbrack {\left( {{\omega \quad t} - {\frac{2\pi}{\lambda}n_{+}d}} \right)} \right\rbrack}} + {\frac{1}{2}\begin{pmatrix}i \\1\end{pmatrix}{{\exp \left\lbrack {\left( {{\omega \quad t} - {\frac{2\pi}{\lambda}n_{-}d}} \right)} \right\rbrack}.}}} & (12)\end{matrix}$

[0076] This can be written as $\begin{matrix}{\frac{1}{2}\exp \left\{ {\left\lbrack {{\omega \quad t} - {\left( {n_{+} + n_{-}} \right)\frac{\pi \quad d}{\lambda}}} \right\rbrack} \right\} \left\{ {{\begin{pmatrix}{- i} \\1\end{pmatrix}{\exp \left\lbrack {{\left( {n_{-} - n_{+}} \right)}\frac{\pi \quad d}{\lambda}} \right\rbrack}} + {\begin{pmatrix}i \\1\end{pmatrix}{\exp \left\lbrack {{\left( {n_{+} - n_{-}} \right)}\frac{\pi \quad d}{\lambda}} \right\rbrack}}} \right\}} & (13)\end{matrix}$

[0077] To interpret the physical meaning of equation (13), the equationis simplified by defining$\phi = {{\frac{\pi \quad d}{\lambda}\left( {n_{+} + n_{-}} \right)\quad {and}\quad \theta} = {\frac{\pi \quad d}{\lambda}{\left( {n_{-} - n_{+}} \right).}}}$

[0078] With these definitions, equation (13) can be written as$\begin{matrix}{{^{\quad {({{\omega \quad t} - \phi})}}\left\lbrack {{\begin{pmatrix}0 \\1\end{pmatrix}\left( \frac{^{\quad \theta} + ^{{- }\quad \theta}}{2} \right)} + {\begin{pmatrix}1 \\0\end{pmatrix}\left( \frac{^{\quad \theta} - ^{{- }\quad \theta}}{2i} \right)}} \right\rbrack} = {^{{({{\omega \quad t} - \phi})}}\left\lbrack {{\begin{pmatrix}0 \\1\end{pmatrix}\cos \quad \theta} + {\begin{pmatrix}1 \\0\end{pmatrix}\sin \quad \theta}} \right\rbrack}} & (14)\end{matrix}$

[0079] Equation (14) describes a plane-polarized wave with thepolarization direction at an angle $\begin{matrix}{\theta = {\frac{\pi \quad d}{\lambda}\left( {n_{-} - n_{+}} \right)}} & (15)\end{matrix}$

[0080] The incident plane-polarized wave has had its plane ofpolarization rotated through an angle θ. This also means that the right-and left-circularly polarized waves have phase delay (φ−θ) and (φ+θ),respectively, after the waves pass through the θ-degree Faraday rotator.

[0081]FIG. 9 shows a schematic cross section of a Gires-Tournoisinterferometer (GTI). The Gires-Tournois interferometer 60 has a firstmirror 62 which is partially reflective, having reflectivity R₁, and asecond mirror 64, which is highly reflective, having reflectivity R₀.The reflectivity R₀ of the second mirror is approximately 100%. Thepartially reflective mirror 62 is spaced apart from and parallel to thehighly reflective mirror 64. The space between the mirrors is the cavity66 and the distance between the two mirrors is d. The first mirror 62provides a single input/output port that allows light to be launchedinto and out of the cavity 66. The spacers 68 are made of ultra-lowexpansion material. The GTI is one of the best candidates for use as aninterferometer in an interleaver because the amplitude response of a GTIis flat (i.e. independent of wavelength). The phase response of the GTIis periodic in λ and is given by $\begin{matrix}{{{\psi (\lambda)} = {{- 2}{\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}}{\tan \left( \frac{2\pi \quad d}{\lambda} \right)}} \right\rbrack}}},} & (16)\end{matrix}$

[0082] where λ is the wavelength, R₁ is the power reflectivity of thefirst mirror, d is the length of the cavity, and$\frac{2\pi \quad d}{\lambda}$

[0083] is the single pass phase delay in the GTI cavity.

[0084]FIG. 10 is a graph of equation (16) showing the phase responseψ(λ) of a GTI as a function of wavelength λ.

[0085]FIG. 11 is a schematic drawing showing a cross section of a GTIFR80 in accordance with the present invention. The GTIFR 80 includes a GTI60 having a first mirror 62 and a second mirror 64. The first mirror 62is partially reflective and has reflectivity R₁. The second mirror 64 ishighly reflective and has reflectivity R₀ of about 100%. The secondmirror 64 is spaced apart from and parallel to the first mirror 62, thedistance between the mirrors being the distance d in FIG. 11. Within thecavity 66 between the mirrors, there is a θ-degree Faraday rotator 82.In front of the GTI there is an a degree Faraday rotator 84. After theright-circularly polarized and left circularly polarized waves arereflected from the GTI with θ-degree Faraday rotator inside, the phasedelays are $\begin{matrix}{{{\psi_{R,L}(\lambda)} = {{- 2}{\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}}{\tan \left( {\frac{2\quad \pi \quad d}{\lambda} + {\phi \mp \theta}} \right)}} \right\rbrack}}},} & (17)\end{matrix}$

[0086] where $\frac{2\quad \pi \quad d}{\lambda}$

[0087] in equation (16) is replaced by$\left( {\frac{2\quad \pi \quad d}{\lambda} + {\phi \mp \theta}} \right),$

[0088] because the θ degree FR in the GTI causes additional phase delays(φ∓θ) for the right-circularly polarized and left-circularly polarizedwaves that travel into the cavity of the GTI. When the right-circularlypolarized wave and the left-circularly polarized wave travel through theα-degree FR from left to right, then reflect back from the GTI, with theθ degree FR inside, and pass through the α degree FR from right to left,the total phase delays for the right and left circularly polarized wavesare $\begin{matrix}{\psi_{R,L} = {{{- 2}{\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}}{\tan \left( {\frac{2\quad \pi \quad d}{\lambda} + {\phi \mp \theta}} \right)}} \right\rbrack}} \mp {2\quad {\alpha.}}}} & (18)\end{matrix}$

[0089] The phase difference Δφ between the right and left circularlypolarized waves is $\begin{matrix}{{\Delta \quad {\varphi (\lambda)}} = {{{- 2}{\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}}{\tan \left( {\frac{2\quad \pi \quad d}{\lambda} + \phi - \theta} \right)}} \right\rbrack}} + {2\quad {\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}}{\tan \left( {\frac{2\quad \pi \quad d}{\lambda} + \phi + \theta} \right)}} \right\rbrack}} - {4\quad {\alpha.}}}} & (19)\end{matrix}$

[0090] The effective length of the GTI cavity is defined as L so that$\begin{matrix}{\frac{2\quad \pi \quad L}{\lambda} = {\frac{2\quad \pi \quad d}{\lambda} + {\phi.}}} & (20)\end{matrix}$

[0091] Using equation (20) in equation (19), equation (19) becomes$\begin{matrix}{{\Delta \quad {\varphi (\lambda)}} = {{{- 2}{\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}}{\tan \left( {\frac{2\quad \pi \quad L}{\lambda} - \theta} \right)}} \right\rbrack}} + {2\quad {\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}}{\tan \left( {\frac{2\quad \pi \quad L}{\lambda} + \theta} \right)}} \right\rbrack}} - {4\quad {\alpha.}}}} & (21)\end{matrix}$

[0092] The values of R₁, θ, α, and L in equation (21) must be such thatΔφ(λ) meets the requirements set forth herein above with regard to FIG.6 and equations (7) and (8). The effective length L of the GTI cavity isdetermined by the channel spacing of the interleaver. For a 50 GHzinterleaver, L is 1.5 mm. θ is a value of phase shift of the curve ψ(λ)(equation (16) in FIG. 10. If ψ_(R)(λ) and ψ_(L)(λ) are offset from eachother by π/2, as shown in FIG. 12, Δφ is symmetric for odd and evenchannels. This requires that 2θ=π/2 or θ=π/4.

[0093] Because cosine and sine functions are periodic and one periodextends from −π to +π, Δφ in equations (7) and (8) can be limited to therange −2π to +2π. Select center wavelengths λ₁ and λ₂ of two adjacentchannels so that$\frac{2\quad \pi \quad L}{\lambda_{1}} = {{2m\quad \pi \quad {and}\quad \frac{2\quad \pi \quad L}{\lambda_{2}}} = {{2m\quad \pi} + \frac{\pi}{2}}}$

[0094] where m is a positive integer). Referring to FIG. 7a, Δφ(λ₁) canbe π or −π and Δφ(λ₂) can be 2π, 0 or −2π. Substituting $\begin{matrix}\begin{matrix}{{\theta = {\pi/4}},{\frac{2\quad \pi \quad L}{\lambda_{1}} = {2m\quad \pi \quad {and}\quad \Delta \quad {\varphi \left( \lambda_{1} \right)}}}} \\{= {{\pi \quad {into}\quad {equation}\quad (21)\quad {gives}\quad 4\quad {\tan^{- 1}\left\lbrack \frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}} \right\rbrack}} - {4\quad \alpha}}} \\{= {\pi.}}\end{matrix} & (22)\end{matrix}$

[0095] Substituting $\begin{matrix}\begin{matrix}{{\theta = {\pi/4}},{\frac{2\quad \pi \quad L}{\lambda_{2}} = {{2m\quad \pi} + \frac{\pi}{2}}},{{and}\quad \Delta \quad {\varphi \left( \lambda_{2} \right)}}} \\{= {{{- 2}\quad \pi \quad {into}\quad {equation}\quad (21)\quad {gives}\quad 4\quad {\tan^{- 1}\left\lbrack \frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}} \right\rbrack}} + {4\quad \alpha}}} \\{= {2\pi}}\end{matrix} & (23)\end{matrix}$

[0096] Adding and subtracting equations (22) and (23) gives$\begin{matrix}{{\tan^{- 1}\left\lbrack \frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}} \right\rbrack} = {\frac{3}{8}\pi}} & (24) \\{{{and}\quad 8\alpha} = \pi} & (25)\end{matrix}$

[0097] Equations (24) and (25) give R₁=17.2% and α=λ/8 (22.5°). Notethat, for Δφ(λ₁)=−π and Δφ(λ₂)=2π or 0, the values obtained for R₁ and αhave no physical meaning.

[0098] The derivation above gives values for all of the parametersneeded for making a deinterleaver or interleaver. These values are asfollows:

[0099] 1. The reflectivity of the first mirror of the GTI R₁=17.2%

[0100] 2. The reflectivity of the second mirror of the GTI R₀=100%

[0101] 3. θ-degree FR inside the GTI cavity θ=45°

[0102] 4. α-degree FR in front of the GTI α=22.5°

[0103]FIG. 13 is a graph of equation (2 1) showing the phase differenceΔφ(λ), as a function of λ, for the GTIFR of FIG. 11, using these derivedparameter values and L=1.5 mm.

[0104] The phase shift for this interleaver or deinterleaver is given by$\begin{matrix}{{\psi_{1}(\lambda)} = {{{- {\tan^{- 1}\left\lbrack \frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}} \right\rbrack}}{\tan \left( {\frac{2\pi \quad L}{\lambda} - \frac{\pi}{4}} \right)}} - {{\tan^{- 1}\left\lbrack \frac{1 + \sqrt{R_{1}}}{1 - \sqrt{R_{1}}} \right\rbrack}{\tan \left( {\frac{2\pi \quad L}{\lambda} + \frac{\pi}{4}} \right)}}}} & (26)\end{matrix}$

[0105] Substituting equation (21) for Δφ in equations (7) and (8), thesignal output for odd channels$\left( {\cos \frac{{\Delta\varphi}(\lambda)}{2}} \right)^{2}$

[0106] and for even channels$\left( {\sin \frac{{\Delta\varphi}(\lambda)}{2}} \right)^{2}$

[0107] can be calculated. FIG. 14 shows the results of thissubstitution. In FIG. 14 the reflectivity R₁ is 18.5% rather than the17.2% derived above. Changing the reflectivity R₁ from 17.2% to 18.5%increases the isolation at pass band edges of adjacent channels up to 25dB. In FIG. 14, the solid trace 112 represents the odd channels and thedashed trace 114 represents the even channels.

[0108] If, in the above discussion, the plane-polarized basis had beenused instead of the circularly polarized basis, the result would havebeen a λ/4 wave-plate in place of the 45° Faraday rotator inside the GTIand a λ/8 wave-plate in place of the 22.5° Faraday rotator in front ofthe GTI. This arrangement of GTI and wave-plates is described in U.S.Pat. No. 6,169,604, entitled, “NONLINEAR INTERFEROMETER FOR FIBER OPTICDENSE WAVELENGTH DIVISION MULTIPLEXER UTILIZING A PHASE BIAS ELEMENT TOSEPARATE WAVELENGTHS IN AN OPTICAL SIGNAL”.

[0109] Starting with equation (4) the derivation of equations (7)through (10) and (26) was limited to the symmetric case by settingα₁=α₂=α. This limitation and these equations are automatically correctno matter how the Faraday rotators are aligned relative to the directionof polarization of the light that is incident on the Faraday rotators.In the case of wave-plates, however, α₁≠α₂ and the equations are notcorrect when the c-axis of a wave plate is misaligned relative to thedirection of polarization. When α₁≠α₂, the equations become verycomplicated and show distortions in spectrum shape and group delay ofthe NLI based interleaver and deinterleaver.

[0110]FIG. 15 is a schematic drawing of a deinterleaver in accordancewith the present invention. The deinterleaver 120 includes one GiresToumois interferometer with Faraday Rotators, GTIFR 122. The GTIFR 122is as shown in FIG. 11. The input optical signal 124 carries opticalsignals for even and odd channels. Collimator 126 collimates the signalbeam. Walk-off crystal 128 separates the beam from the collimator 126into a vertically polarized beam and a horizontally polarized beam 130.The vertically polarized beam passes through a half-wave plate 132 andemerges from the half-wave plate as a horizontally polarized beam 134.The mirror 136 reflects the two horizontally polarized beams to apolarization beam splitter (PBS) 138. Both horizontally polarizedsignals pass through PBS 138, through 22.5 cut half-wave plate (whichrotates the polarization of the signals a negative 45 degrees), andthrough PBS 144 to GTIFR 122. The signals entering the GTIFR 122 arehorizontally polarized. The signal carrying odd channels is reflectedfrom GTIFR 122 with horizontal polarization and passes through PBS 144,then through garnet 142 which rotates the polarization of horizontallypolarized signals through 45 degrees, and then through 22.5 cuthalf-wave plate 140 which rotates the polarization through another 45degrees, thus making the odd channel signal vertically polarized. PBS138 reflects the vertically polarized odd channel signal. Afterreflection in PBS 138, portion of the vertically polarized odd channelsignal passes through half-wave plate 146 and emerges with horizontalpolarization and enters the walk-off crystal 148. Another portion of thesignal reflected from PBS 138 enters the walk-off crystal directly. Thewalk-off crystal combines the vertically polarized portion with thehorizontally polarized portion to provide an output signal carrying theodd channels to collimator 150.

[0111] The signal carrying even channels emerges from the GTIFR 122 andis reflected by PBS 144. After reflection from PBS 144, the even channelsignal has vertical polarization. After reflection from PBS 144, part ofthe vertically polarized light passes through half-wave plate 152 (wherethe direction of polarization is changed to horizontal) into walk-offcrystal 154 and part goes directly into the walk-off crystal 154.Walk-off crystal 154 combines the two portions to provide an outputsignal carrying the even channels to collimator 156.

[0112] If, in the GTIFR 122 in FIG. 15, the external Faraday rotator isreplaced with a Faraday rotator having magnetic field directionswitched, α becomes −α and so Δφ(λ) in equation (21) changes by π and$\left( {\cos \frac{{\Delta\varphi}(\lambda)}{2}} \right)^{2}$

[0113] becomes$\left( {\sin \frac{{\Delta\varphi}(\lambda)}{2}} \right)^{2}$

[0114] and vice versa. The output signal locations are changed, with anoutput signal for odd channels appearing at collimator 156 and theoutput signal for even channels appearing at collimator 150.

[0115]FIG. 15A is a schematic drawing of an interleaver in accordancewith the present invention. The interleaver 120 of FIG. 15A is the samedevice as the deinterleaver of FIG. 15. The interleaver of FIG. 15Areceives an odd channel input signal via collimator 126 and receives aneven channel signal through collimator 156 and outputs the combinedsignals for odd and even channels at collimator 150.

[0116] The odd channel signal passes through collimator 126 and walk-offcrystal 128. Walk-off crystal 128 divides the odd channel input signalinto a horizontally polarized portion 130 and a vertically polarizedportion which passes through half-wave plate 132 to become horizontallypolarized portion 134. The horizontally polarized odd channel signal isthen reflected in mirror 136 and then passes through PBS 138, 22.5 cuthalf-wave plate 140, garnet 142, and PBS 144 into GTIFR 122. GTIFR 122reflects the horizontally polarized signals of the odd channels.

[0117] The even channel input signal passes through collimator 156 andthrough walk-off crystal 154, which divides the signal into a verticallypolarized portion, which passes, directly to PBS 144, and a horizontallypolarized portion, which passes through half-wave plate 152 andcontinues as vertically polarized to PBS 144. Both vertically polarizedportions of the even channel signal are reflected by PBS 144 into GTIFR122.

[0118] Both the even channel signal and the odd channel signal leave theGTIFR 122 and pass through PBS 144, through garnet 142 which rotates thepolarization of horizontally polarized signals through 45 degrees, andthrough 22.5 cut half-wave plate 140 which rotates the polarizationthrough another 45 degrees, thus making both the odd channel signal andeven channel signal vertically polarized. The vertically polarizedsignals are reflected by PBS 138. A portion of the signals passesthrough half-wave plate 146 and becomes horizontally polarized and thenenters walk-off crystal 148. The remainder of the signals passesdirectly from PBS 138 to walk-off crystal 148. The walk-off crystal 148combines the vertically polarized signal with the horizontally polarizedsignal and outputs an output signal containing odd and even channels viacollimator 150.

[0119]FIG. 16 is a schematic drawing showing a deinterleaver withdispersion compensation, in accordance with the present invention. Theinput signal contains odd channels and even channels. Collimator 126collimates input signal 124. Walk-off crystal 158 separates the inputsignal into a vertically polarized signal and a horizontally polarizedsignal. The half-wave plate 160 converts the horizontally polarizedsignal to vertical polarization so that the input signal is verticallypolarized. The PBS 162 reflects the vertically polarized input signal toquarter wave-plate 164, which transforms the plane-polarized signal to acircularly polarized signal. The circularly polarized signal travels tothe dispersion compensator 166, which changes the phase of the signalsby ψ_(c)(λ). The signal reflected from the dispersion compensator 166passes again through the quarter wave-plate 164, which now transformsthe circularly polarized signal to a plane-polarized signal, polarizedin the horizontal direction. The horizontally polarized signal passesthrough PBS 162, PBS 138, 22.5 cut half-wave plate 140 (which rotatesthe polarization through 45° in a positive sense), garnet 142 (whichrotates the polarization of the signals through 45° in a negativesense), and PBS 144 to GTIFR 122. The signal reflected from GTIFR 122contains a horizontally polarized odd channel signal and a verticallypolarized even channel signal.

[0120] The horizontally polarized odd channel signal passes through PBS144, garnet 142 (which rotates the horizontally polarized signal through45 degrees), and 22.5 cut half-wave plate 140 (which rotates thepolarization another 45°) and emerges as a vertically polarized oddchannel signal, which is reflected by PBS 138. A portion of thevertically polarized signal reflected from PBS 138 passes throughhalf-wave plate 146 and enters walk-off crystal 148 as a horizontallypolarized signal. Another portion of the vertically polarized signalreflected from PBS 138 enters the walk-off crystal directly. Thewalk-off crystal combines the two portions into an odd channel outputsignal which passes through collimator 150.

[0121] The vertically polarized even channel signal from GTIFR 122 isreflected by PBS 144 to walk-off crystal 154. A portion of the signal ischanged to horizontal polarization by half-wave plate 152 beforeentering the walk-off crystal 154. The walk-off crystal combines thehorizontally polarized portion with the vertically polarized portion andoutputs an output signal carrying the even channels through collimator156.

[0122]FIG. 17 is a schematic drawing of a dispersion compensatedinterleaver 200 in accordance with the present invention. Theinterleaver 200 of FIG. 17 differs from the deinterleaver 180 of FIG. 16only in that the garnet 142 is rotated 180° about an axis perpendicularto the page. Thus, if the garnet 142 in FIG. 16 rotates the polarizationof the light by 45° in a positive sense, the garnet 142 in FIG. 17rotates the polarization 45° in a negative sense. Collimator 150collimates the odd channel input signal. The walk-off crystal 148 andhalf-wave plate 146 change the signal to a vertically polarized signal.PBS 138 reflects the vertically polarized input signal for odd channelsto the half-wave plate 140. The odd channel signal passes through thehalf-wave plate 140 which rotates the polarization of the signal by 45°,and through the garnet 142 which rotates the polarization another 45° sothat the odd channel signal becomes horizontally polarized and passesthrough the PBS 144 to the GTIFR 122. The odd channel signal retainshorizontal polarization when reflected by GTIFR 122 and then passthrough PBS 144, through garnet 142 which rotates the horizontallypolarized signal by 45° in one sense, through half-wave plate 140 whichrotates the polarization by 45° in the opposite sense, so thathorizontally polarized signal passes through PBS 138 and 162 to quarterwave-plate 164. Quarter wave-plate 164 transforms the plane-polarizedsignal to a circularly polarized signal. The circularly polarized signaltravels to the dispersion compensator 166 which changes the phase of thesignal by ψ_(C)(λ) as given by equation (28). After reflection by thedispersion compensator 166, the signal passes again through the quarterwave-plate 164, where the circularly polarized signal becomes planepolarized in the vertical direction. The vertically polarized signal isreflected by PBS 162 to half wave-plate 160 and walk-off crystal 158.The half-wave plate 160 changes the polarization direction of a portionof the signal to the horizontal direction and the walk-off crystalcombines this with the remainder vertically polarized portion and sendan output signal carrying the odd channels to collimator 126, the commonoutput port.

[0123] The input signal carrying even channels is changed to avertically polarized signal by the action of the walk-off crystal 154and the half wave-plate 152 and is then reflected by PBS 144 into GTIFR122. After the signal has been reflected by the GTIFR, the polarizationof the signal is horizontal and the signal travels to the dispersioncompensator and from there to the common out port in the same manner asthe odd channel signal as described above. The common output port,collimator 126 outputs the combined signals for all channels withdispersion compensation.

[0124] In the de-interleaver of FIG. 16 and in the interleaver of FIG.17, a Faraday rotator with external magnetic field may replace thegarnet 142. Changing the direction of the external magnetic field issufficient to change from interleaver to de-interleaver or vice versa.

[0125] The total phase shift of the dispersion compensated deinterleaverof FIG. 16 and the dispersion compensated interleaver of FIG. 17 isgiven by

ψ_(T)(λ)=ψ₁(λ)+ψ_(C)(λ)   (27)

[0126] where ψ₁(λ) is the phase of the output signals from the GTIFRinterleaver or deinterleaver as given by equation (26) withoutdispersion compensation, and ψ_(C)(λ) is the phase response of thedispersion compensator 166. The dispersion compensator 166 is a GiresTournois interferometer as shown in FIG. 9 for which the phase responseis given by $\begin{matrix}{\psi_{C} = {{- 2}\quad {\tan^{- 1}\left\lbrack {\frac{1 + \sqrt{R_{2}}}{1 - \sqrt{R_{2}}}{\tan \left( \frac{2\pi \quad d}{\lambda} \right)}} \right\rbrack}}} & (28)\end{matrix}$

[0127] The group delay of the dispersion compensator 166 is given by$\begin{matrix}{{{\tau (\lambda)} = {\frac{0.01\lambda^{2}}{6\pi}\frac{d\quad {\psi_{C}(\lambda)}}{d\quad \lambda}}},} & (29)\end{matrix}$

[0128] and the dispersion D(A) in units of ps/nm is given by$\begin{matrix}{{D(\lambda)} = {10^{- 3}{\frac{d\quad {\tau (\lambda)}}{d\quad \lambda}.}}} & (30)\end{matrix}$

[0129]FIG. 18 is a graph of the group delay τ(λ) in picoseconds versuswavelength λ in microns, as given by equation (29) for a dispersioncompensator 166.

[0130]FIGS. 19 and 20 are graphs showing the group delay in picoseconds,and dispersion in picoseconds per nanometer, respectively, for aninterleaver (or deinterleaver) as in FIG. 15, where the channel spacingis 50 GHz, the reflectivity R₁=18.5% and L=1.5 mm.

[0131] From FIG. 20, the dispersion is ±40 ps/nm in bandwidth of ±0.08nm (±10 GHz).

[0132]FIGS. 21 and 22 are graphs showing the group delay and dispersion,respectively, for a de-interleaver as in FIG. 16 and for an interleaveras in FIG. 17, wherein the channel spacing is 50 GHz, the reflectivityR₁=18.5% and L=1.5 mm, and wherein the dispersion compensator is a GTIas in FIG. 9 with cavity length d=3 mm and reflectivity of the partiallyreflective mirror R₂=0.28%.

[0133] In the present invention, the value R₁ may be in the range fromabout 17.2% to about 19.0%, while the value of 18.5% is preferred.Likewise, in the present invention, the value of R₂ may be in the rangefrom about 0.28% to about 0.40%, while the value of 0.28% is preferred.A garnet is the preferred type of 45-degree Faraday and also thepreferred type of 22.5-degree Faraday rotator, however, any othersuitable type of Faraday rotators may be used in the present invention.

[0134] From FIG. 22, the dispersion is ±4.7 ps/nm in ±0.08 nm or ±10 GHzbandwidth. This is only 12% of the dispersion of the interleaver withoutdispersion compensation.

[0135] Exemplary embodiments of the present invention have beendescribed herein. These are intended to be illustrative and notrestrictive. Thus, the present invention is capable of many variationsin detailed implementation, which may be derived from the descriptioncontained herein by a person of ordinary skill in the art. All suchvariations are considered to be within the scope of the appended claims.

What is claimed is:
 1. An interleaver comprising: a GTIFR interferometerfor receiving a plane polarized input signal containing even channelsignals and for receiving a plane polarized input signal containing oddchannel signals and for reflecting a phase shifted signal containing theeven channel signals and the odd channel signals; and a polarizationbeam splitter for receiving the phase shifted signal from the GTIFR andfor producing a plane polarized output signal containing both the oddchannel signals and the even channel signals, wherein the direction ofpolarization of the plane polarized input signal containing even channelsignals is orthogonal to the direction of polarization of the of theplane polarized input signal containing odd channel signals, and whereinthe direction of polarization of the output signal is the same as one ofthe directions of polarization of the input signals.
 2. The interleaverof claim 1, further comprising: a Gires-Tournois interferometerdispersion compensator, wherein the output signal from the polarizationbeam splitter is reflected from the Gires-Tournois interferometerdispersion compensator to produce a dispersion compensated output signalcontaining even channel signals and odd channels signals.
 3. Theinterleaver of claim 2, wherein the Gires-Tournois interferometerdispersion compensator comprises a partially reflective mirror havingreflectivity between about 0.28% and about 0.40%.
 4. The interleaver ofclaim 2, wherein the Gires-Tournois interferometer dispersioncompensator comprises a partially reflective mirror having reflectivityof about 0.28%.
 5. The interleaver of claim 1, further comprising: meansfor removing the polarization from the plane polarized output signal. 6.The interleaver of claim 5, wherein the means for removing thepolarization from the output signal comprises a half-wave plate foraltering the direction of polarization of a portion of the planepolarized output signal by ninety degrees and a walk-off crystal forcombining the remainder of the plane polarized output signal with thealtered portion to produce an unpolarized output signal that containseven channel signals and odd channel signals.
 7. A deinterleavercomprising: a GTIFR interferometer for receiving a plane polarized inputsignal containing even channel signals and odd channel signals and forreflecting a phase shifted signal containing the even channel signalsand the odd channel signals; and a polarization beam splitter forreceiving the phase shifted signal from the GTIFR and for producing aplane polarized output signal containing the even channel signals andfor producing a separate plane polarized output signal containing theodd channel signals, wherein the direction of polarization of the planepolarized output signal containing even channels is perpendicular to thedirection of polarization of the plane polarized output signalcontaining odd channels.
 8. The deinterleaver of claim 7, furthercomprising: a Gires-Tournois interferometer dispersion compensator fromwhich the input signal containing the even channel signals and the oddchannel signals is reflected before being reflected by the GTIFR.
 9. Thedeinterleaver of claim 8, wherein the Gires-Tournois dispersioncompensator comprises a partially reflective mirror having reflectivityof between about 0.28% and about 0.40%.
 10. The deinterleaver of claim8, wherein the Gires-Tournois dispersion compensator comprises apartially reflective mirror having reflectivity of about 0.40%.
 11. Thedeinterleaver of claim 7, further comprising means for removingpolarization from the plane polarized output signal containing evenchannel signals.
 12. The deinterleaver of claim 11, wherein the meansfor removing polarization from the plane polarized output signalcontaining even channels comprises a half-wave plate for altering thedirection of polarization of a portion of the output signal containingeven channel signals by ninety degrees and a walk-off crystal forcombining the remainder of the plane polarized output signal containingeven channels with the altered portion to produce an unpolarized outputsignal containing even channels.
 13. The deinterleaver of claim 7,further comprising: means for removing polarization from the planepolarized output signal containing odd channel signals.
 14. Thedeinterleaver of claim 13, wherein the means for removing polarizationfrom the plane polarized output signal containing odd channels comprisesa half-wave plate for altering the direction of polarization of aportion of the output signal containing odd channel signals by ninetydegrees and a walk-off crystal for combining the remainder of the planepolarized output signal containing odd channels with the altered portionto produce an unpolarized output signal containing odd channels.